1. © Crown copyright Using ensemble rainfall predictions in a countrywide flood forecasting model in Scotland Why Predict? The value of prediction in hydrological sciences and policy Michael Cranston, EGU General Assembly 2012

2. © Crown copyright Acknowledgements Bob Moore, Steve Cole, Alice Robson, Vicky Bell and Phil Howard of CEH Wallingford Alex Minett and Marcel Ververs of Deltares Peter Buchanan and Alan Motion of Met Office Richard Maxey, Amy Tavendale, Lauren McLean and Rachel Geldart of the Flood Forecasting and Warning Section of SEPA

3. © Crown copyright Aims What policy is driving our flood forecasting developments What are the key challenges in flood forecasting uncertainty and how are we tackling this through science developments and applications What have we learned from recent operational experience of use of probabilistic flood forecasts

4. © Crown copyright Flood Risk Management (Scotland) Act 2009 Science to improve flood forecasting Flood warning and risk based communication

5. © Crown copyright Flood Warning Policy “…SEPA will aim to reduce the impact of flooding through the provision of actively disseminated, reliable and timely flood warnings to registered users of a national flood warning service. …and SEPA will aim to provide an effective flood warning service and to reduce the impact of flooding from all sources.” SEPA (2010) Statement of Intent for Implementing Flood Warning Duties under the Flood Risk Management Scotland Act

6. © Crown copyright Service Expectations “…public expectations around flood warning systems can never be fully met.” Association of Chief Police Officers (2008) Scottish Parliament Flooding and Flood Management Review, RAE Committee 2 nd Report.

7. © Crown copyright Hydro-meteorological forecasting “…the committee recommends that the Scottish Government ensures that SEPA has the necessary funding to enable it to collaborate with the Met Office to provide an effective flood warning system for all types of floods.” Scottish Government (2008) Scottish Parliament Flooding and Flood Management Review, RAE Committee 2 nd Report.

8. © Crown copyright National guidance on expected flood risk based on risk based approach (likelihood and impact). Raising vigilance and awareness. Lead time T +5 days Community level warning when flooding is expected – Immediate Action Required. Lead time T+ 3-6 hours Administrative authority level scale alerts when flooding is possible – be prepared. Lead time T +24 – 36 hours

9. © Crown copyright Flood forecasting on a national scale – Is this achievable?

10. Grid-to-Grid (G2G) Distributed Model Uses spatial datasets on terrain, soil/geology, land-cover Responds to spatial variation of rainfall input Distributed Model Structure Subsurface flow-routing Saturation-excess surface runoff Drainage River Surface flow-routing Precipitation Evaporation Return flow River flow Runoff- producing soil column Acknowledgement: Moore et. al, CEH Wallingford, 2011

11. Factors shaping runoff production in G2G G2G Model: runoff shaping Soil properties (for each HOST class) Terrain/slope Steep slopes → Shallow storage Shallow slopes → Deep storage Soil Moisture Deficit (modelled) Gridded rainfall G2G Runoff Production Urban/Suburban coverage Increase runoff (by decreasing soil depth) Acknowledgement: Moore et. al, CEH Wallingford, 2011

12. G2G Model Performance: R 2 Efficiency 2008 Water Year (all configured sites) Acknowledgement: Moore et. al, CEH Wallingford, 2011

13. © Crown copyright Quantitative Precipitation Forecasts Deterministic Grid-to-Grid utilises best available (greatest resolution) weather prediction Nowcast (2 km) UK4 (4 km) North Atlantic European (12 km) Global (25 km) Ensemble Grid-to-Grid utilises current operational MOGREPS-R (18 km)

14. Flood Forecasting and Warning MOGREPS members T +54 UKPP, UK4, NAE, GM deterministic T +144 P > 75% P > 25%

15. © Crown copyright Countrywide assessment of flood risk Probability of exceeding flow threshold Q(T) Good indication of flooding hotspots Operational Utility 4 th January 2012

16. © Crown copyright

18. Flood Warning Threshold (Q) Peak Flow (Q) Warning Status G2G Deterministic Peak (Q) G2G MOGREPS Median (Q) G2G % ile FW Crossing 10539520511875% 112321281224100% 7066596225% 1261109311625%

19. © Crown copyright

20. Value of countrywide forecasting Distributed model provides spatial representation of forecast risk with lead time of 1-2 days, especially when using probabilities Greater accuracy of river flow forecast when using higher resolution deterministic QPF models Ensemble-based MOGREPS-R forecasts provide objective assessment of risk on a national scale

21. © Crown copyright National guidance on expected flood risk based on risk based approach (likelihood and impact). Raising vigilance and awareness. Lead time T +5 days Community level warning when flooding is expected – Immediate Action Required. Lead time T+ 3-6 hours Administrative authority level scale alerts when flooding is possible – be prepared. Lead time T +24 – 36 hours

22. © Crown copyright Application for flood warning “…(the forecast) provided a useful heads up, not only for the potential for high levels especially within the Irvine but also the confidence that could be attributed to these forecasts by seeing the spread of the predicted hydrographs. …it did mean that … I was more prepared.” David Fadipe (2012) Flood Warning Duty Officer response to consultation on use of probabilistic forecasts, pers. comms.

23. © Crown copyright Conclusions Greater than ever expectations on flood warning delivery based on policy drivers Significant advances in hydrometeorology with specialists working together Use of ensemble prediction good at countrywide scale but higher resolution (storm scale) models needed for flood warning application Understanding uncertainties helps risk based flooding guidance and communication